While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less.
For resist materials for use with a KrF excimer laser, polyhydroxystyrene having a practical level of transmittance and etching resistance is, in fact, a standard base resin. For resist materials for use with an ArF excimer laser, polyacrylic or polymethacrylic acid derivatives and polymers comprising cycloaliphatic compounds in the backbone are under investigation. In either case, the basic concept is that some or all of alkali soluble sites of alkali soluble resin are protected with acid labile groups. The overall performance of resist material is adjusted by a choice from among a variety of acid labile groups.
Exemplary acid labile groups include tert-butoxycarbonyl (JP-B 2-27660), tert-butyl (JP-A 62-115440 and J. Photopolym. Sci. Technol. 7 [3], 507 (1994)), 2-tetrahydropyranyl (JP-A 2-80515 and JP-A 5-88367), and 1-ethoxyethyl (JP-A 2-19847 and JP-A 4-215661). While it is desired to achieve a finer pattern rule, none of these acid labile groups are deemed to exert satisfactory performance.
More particularly, tert-butoxycarbonyl and tert-butyl are extremely less reactive with acids so that a substantial quantity of energy radiation must be irradiated to generate a sufficient amount of acid in order to establish a difference in rate of dissolution before and after exposure. If a photoacid generator of the strong acid type is used, the exposure can be reduced to a relatively low level because reaction can proceed with a small amount of acid generated. However, in this event, the deactivation of the generated acid by air-borne basic substances has a relatively large influence, giving rise to such problems as a T-top pattern. On the other hand, 2-tetrahydropyranyl and 1-ethoxyethyl are so reactive with acids that with only the acid generated by exposure, elimination reaction may randomly proceed without wait until heat treatment, resulting in substantial dimensional changes between exposure and heat treatment/development. Where these groups are used as protective groups for carboxylic acid, they have a low inhibiting effect to alkali dissolution, resulting in a high rate of dissolution in unexposed areas and film thinning during development. If highly substituted polymers are used to avoid such inconvenience, there results an extreme drop of heat resistance. These resins fail to provide a difference in rate of dissolution before and after exposure, resulting in resist materials having a very low resolution.
In case where 2-adamantyloxymethyl is used as the protective group for carboxylic acid, its performance is good with respect to sensitivity and resolution, but such a protective group of methylene acetal type gives off formaldehyde having a low boiling point after deprotection, leaving the risk of lens contamination by outgassing. While a further reduction of pattern feature size is being demanded, there is a need to have a resist material which is not only satisfactory in sensitivity, resolution, and etch resistance, but also minimized in outgassing.